Tupile is the "so-called 'sanctuary planet' (probably several planets) for defeated Houses of the Imperium. Location(s) known only to the Guild and maintained inviolate under the Guild Peace." Tupile is said to be "outside the Imperium", though this is probably not meant in the literal sense that it is physically far beyond the borders of colonized space. Rather, because the Spacing Guild controls space travel to and from Tupile, its location (or locations) could easily be anywhere within the Imperium.
Read more about this topic: List Of Dune Planets